1. Field of Invention
The present invention relates to a connecting structure of a printed circuit board. More particularly, the present invention relates to a connecting structure of a flexible printed circuit board of a liquid crystal display (LCD) module.
2. Related Art
Recently, because the objectives of the electronic products are light, thin, short and small, many electronic devices develop according to the objectives, and the using of the flexible printed circuit board totally accord with the requirement of the modern technology. In addition, due to the unique flexibility of the flexible printed circuit board, the three-dimensional stereo wiring may be easily accomplished together with the spatial configuration, such that the flexible printed circuit board is widely used, particularly, it is usually applied in the LCD module, so as to serve as the bridge electrically connecting different circuit elements.
Referring to FIG. 1, it is a schematic view of the connecting structure of the flexible printed circuit board of the conventional LCD module. As shown in FIG. 1, the LCD module 10 mainly includes a LCD panel 16, a backlight module 17, a first flexible printed circuit board 13, a second flexible printed circuit board 14 and a mainboard printed circuit board 18.
The LCD panel 16 is connected to the mainboard printed circuit board 18 through the first flexible printed circuit board 13, so as to achieve the signal transmission. The backlight module 17 uses the second flexible printed circuit board 14 as the carrier of the circuit. Usually, the second flexible printed circuit board 14 is connected to the first flexible printed circuit board 13 by welding, and the first flexible printed circuit board 13 is connected to the mainboard printed circuit board 18.
A solder surface 131 is formed on the first flexible printed circuit board 13. The second flexible printed circuit board 14 has an extending portion 15, and a conducting surface 12 is formed on the extending portion 15. Through the welding of the conducting surface 12 and the solder surface 131, the effect of the electrical connection between the LCD panel 16 and backlight module 17 is achieved.
Usually, the first flexible printed circuit board 13 is a double side printed circuit board structure, and its connecting position with the second flexible printed circuit board 14 is at a solder surface 131 on one side of the first flexible printed circuit board 13. The second flexible printed circuit board 14 mainly adopts the double side printed circuit board structure, both of the two sides have a conducting surface 12, and the two conducting surfaces 12 may be welded with the solder surface 131.
As shown in FIGS. 2A and 2B, a sectional view and a top view of the conventional connecting structure of the first flexible printed circuit board of the double side printed circuit board structure and the second flexible printed circuit board of the double side printed circuit board structure are shown. A plurality of first pads 21 and a plurality of second pads 22 are respectively disposed on the solder surface 131 and the conducting surface 12, wherein the area of the first pad 21 may be larger than, equal to or less than the area of the second pad 22. In the figures disclosed by this embodiment, the area of the first pad 21 is larger than the area of the second pad 22. During the welding operation, according to the structural requirements, the second pad 22 on any conducting surface 12 is selected to correspond to the first pad 21 on the solder surface 131 to get into contact with each other. Because the area of the first pad 21 is larger than the area of the second pad 22, the first pad 21 still leaves a protruding portion 21a after contacting, the solder 23 may be welded with the non-contacted second pad 22 through the protruding portion 21a. Moreover, a part of the solder located on the first pad 21 spills to the second pad 22 on the other side through the plated through holes (PTHs) formed on the second pad 22.
Although the above bonding structure is convenient for the bonding process, the second flexible printed circuit board of the double side printed circuit board structure is expensive, and the production cost is increased correspondingly.
Therefore, in order to save the material cost, the conventional method changes to adopt the second flexible printed circuit board of the single side printed circuit board structure, according to different connecting manners, the connecting structures usually have the following two configurations.
As shown in FIGS. 3A and 3B, a sectional view and a top view of the conventional first connecting structure of the first flexible printed circuit board of the double side printed circuit board structure and the second flexible printed circuit board of the single side printed circuit board structure are shown. The second pads 22 disposed on the second flexible printed circuit board and the first pads 21 disposed on the first flexible printed circuit board are not directly contacted, that is, the second pads 22 and the first pads 21 are disposed on the same side of the flexible printed circuit board. During the welding operation, the solder 23 is used to connect the protruding portion 21a of the first pad 21 and the second pad 22. However, whether the strength of the welding structure is appropriate becomes a hidden trouble of the product yield. Usually, in order to make the welding structure have the substantive bonding strength, it is necessary to provide enough amount of solder, thus the tin bead is usually too big, causing the problems on the product assembling or the appearance dimension. On the contrary, if the amount of the solder is not enough, the bonding section is too small, it is impossible to provide enough bonding strength, and the tin crack is easily generated because of the external force, thus resulting in the poor electrical connecting.
As shown in FIGS. 4A and 4B, a sectional view and a top view of the second conventional connecting structure of the first flexible printed circuit board of the double side printed circuit board structure and the second flexible printed circuit board of the single side printed circuit board structure are shown. Different from the connecting method shown in FIGS. 3A and 3B, the second pad 22 and the first pad 21 are bonded by contacting directly. In order to make the solder 23 sandwiched between the solder surface 131 and the conducting surface 12 achieve the melting state, it is necessary to increase the welding temperature and to prolong the welding time. However, the heat energy must be transmitted though the protective layer of the flexible printed circuit board, usually the circuit board is burnt and becomes black. Because the solder 23 is located between the two pads, during the welding process, it is not easy to observe the melting condition of the solder 23, so usually the false bonding occurs, resulting in the poor electrical connecting, and thereby influencing the product yield.